The pre-stressing of reinforcing elements within concrete beams is utilized to impart high bending resistance to a relatively low mass, lightweight beam. Such a structure is typified in the disclosure of U.S. Pat. No. 3,384,522.
Also, it is known to reinforce extruded or molded plastic members with imbedded elements wherein the elements provide the member with a higher strength, and the plastic material provides resistance to corrosion, an attractive appearance, and protects the element. Disclosures of this type are shown in U.S. Pat. Nos. 3,352,060 and 3,470,598.
Glass fiber reinforced members utilizing thermosetting resins may be formed by a process commonly called "pultrusion" wherein the element is formed by a plurality of glass fiber filaments wetted with a thermosetting resin, pulled through a die for shaping, and the resin impregnated filaments are cured and the member cut to length. As the filaments are pulled through the processing apparatus the filaments are maintained under tension during curing and members so constructed have a high resistance to tensile forces. U.S. Pat. No. 3,657,040 discloses apparatus of this type.
It is also known to utilize glass fiber elements in conjunction with shaped plastic structural members wherein the elements are imbedded within the plastic member to improve the bending resistance of the member, and in the assignee's U.S. Pat. No. 3,574,104 a disclosure of a construction member of this type is made.
The imbedding of reinforcing elements within thermoplastic members formed by extrusion is also known as shown in U.S. Pat. No. 4,740,405, and in the production of such a member a crosshead die may be used to imbed the elements within the thermoplastic material. However, while such known constructions as exemplified by the aforementioned patent produce a member which is reinforced by the elements, the elements are not pre-stressed and the resistance to tension forces applied to the member is limited.
Previously, the formation of a thermoplastic extruded member utilizing imbedded pre-stressed tensioned elements has not been accomplished, and it is an object of the invention to provide such an extruded thermoplastic member having greater resistance to bending than similar members not utilizing pre-stressed elements.
Another object of the invention is to provide a structural member formed of extruded thermoplastic material wherein the material is extruded about tensioned reinforcing elements and the initial tension on the elements is not relieved until the member material has cured wherein integral bonding between the material of the member and elements is achieved and the elements impose compressive forces upon the thermoplastic material.
An additional object of the invention is to provide a process for forming a reinforced thermoplastic structural member utilizing pre-stressed reinforcing elements wherein the pre-stressing of the elements, the extrusion of the thermoplastic configuration, and the curing of the structural member is formed in a continuous process.
A further object of the invention is to provide a thermoplastic extruded structural member reinforced by glass fibers imbedded in a thermosetting resin to form reinforcing elements wherein the impregnation of the glass fibers with the resin and the formation of the elements is continuous with the formation of the extruded structural member whereby heating and curing of the elements and member is continuous to efficiently utilize heat, and improve bonding between the elements and the member material.
In the practice of the invention the raw reinforcing material, which usually takes the form of glass fiber filaments, or a woven or non-woven web formed of glass fiber filaments is preheated and then passed through a thermosetting resin. The resin is wiped from the glass fiber material and shaped to form a particular reinforcing element configuration and the reinforcing element passes through a heated curing chamber wherein the element is partially cured and substantially hardened. The element then passes into the crosshead die of extrusion apparatus which includes a die configuration to define the outer structural member form and internal guides within the crosshead maintain the reinforcing elements in a predetermined relationship to each other and the form of the structural member being extruded through the crosshead die as formed of a thermoplastic material.
The structural member leaving the extrusion press crosshead will be of a configuration as determined by the crosshead die opening and contain the reinforcing material therein. This assembly is immediately exposed to a quenching process, either water or air, to lower the temperature of the thermoplastic structural member. The formed and hardened structural member then passes through the traction device which has been tensioning the structural member bar and the reinforcing elements throughout the entire process, and thereafter, the structural member bar being formed is cut into desired lengths and thereafter punched or otherwise processed.
The fact that the reinforcing elements are tensioned as they pass through the extrusion press, and while the thermoplastic structural member body is curing and hardened, causes the reinforcing elements to be tensioned or pre-stressed as imbedded within the final structural member. The location of the elements relative to each other and the form of the structural member is predetermined with respect to the lateral bending forces to be imposed upon the structural member so as to give the resultant structural member the greatest resistance to bending in the appropriate direction.
The reinforcing elements are, preferably, fully imbedded within the thermoplastic material so as to be hidden from view, and as the thermoplastic material can be colored or otherwise treated in an attractive manner, the practice of the invention permits aesthetically acceptable structural members of a lightweight, complex configuration to be formed which are also capable of resisting significant deflection under lateral bending forces.
The exterior surface of the reinforcing elements may be roughened, or otherwise treated, so as to improve the bonding or adhesion between the tension elements and the thermoplastic material, and structural members constructed in accord with the invention are capable of significantly greater resistance to bending as compared with non-reinforced members, or thermoplastic members having reinforcing elements located therein which are not pre-stressed or installed under tension.